The mechanism by which erythropoietin interacts with its target cells in unknown but recent studies suggest that the hormone may exert its primary effect at the level of the plasma membrane. To test this hypothesis, we plan to study the influence of well defined membrane-active agents on erythropoietin-mediated erythroid colony-formation in vitro. In particular, we will employ agents which alter transmembrane cation fluxes by interacting with specific membrane-bound ion pumps, or by selectively altering membrane permeability to certain ions. We will also investigate whether tumor-promoting phorbol esters which stimulate myelopoiesis in vitro while inhibiting burst formation, exert their effects as surrogate regulatory factors at the level of the plasma membrane. The ability of the reporter molecule Merocyanine 540 to serve as a membrane marker for both normal and transformed cells will be exploited to evaluate differences in cell membrane structure and function during the commitment and differentiation of both normal and neoplastic hematopoietic progenitor cells. We have recently observed that hematopoietic progenitor cells exhibit a differential sensitivity to inhibition of proliferation by ethanol and acetaldehyde, both of which are known to be membrane-active agents. We now plan to investigate the mechanism by which ethanol and acetaldehyde exert their effects on hematopoietic progenitor cell proliferation. Finally, we want to determine whether continuous production of erythropoietin can be achieved by fusing rodent kidney cells programmed to produce erythropoietin with mouse myeloma cells.